This invention relates to an echo canceller system for cancelling an echo signal in a long distance telephone communication circuit and a conference telephone system.
An echo canceller system has been used for controlling or cancelling echos in a long distance circuit, a broadcasting telephone set, a bidirectional repeater or the like.
In the prior art echo canceller system, however, when the echo signal is mixed with or superposed upon such a disturbance signal as a speech signal or a noise signal and is input to a transmission side, the measuring accuracy of the echo circuit will be decreased. Moreover, the degree of echo cancellation is decreased to a value determined by the ratio between the echo signal and the level of the external disturbance signal which are input to the transmission side.
To prevent such a decrease in the degree of echo cancellation caused by such factors, an echo canceller system has been proposed in which a signal remaining after cancellation (called a residual signal) is used to detect a double talk.
One example of such a prior art echo canceller system will be described later in detail. But such a system has a number of difficulties.